KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web. From 0.11.0 until 0.18.2, KaTeX uses ordinary JavaScript property access for the renderer options object, the trust setting, default and processor setting metadata, and namespace lookup and group restoration, allowing inherited properties to be treated as explicitly supplied values. When Object.prototype is already polluted or an attacker controls the options object's prototype, attacker-controlled mathematical expressions can use an inherited trust value to enable trusted rendering and produce links capable of user-interaction cross-site scripting or loading attacker-selected external resources in a consuming application that inserts unsanitized KaTeX output into a page. KaTeX does not itself create the prototype pollution, and rendering an expression alone does not execute script. This issue is fixed in version 0.18.2.

KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web. KaTeX users who render untrusted mathematical expressions with renderToString could encounter malicious input using \htmlData that runs arbitrary JavaScript, or generate invalid HTML. Users are advised to upgrade to KaTeX v0.16.21 to remove this vulnerability. Users unable to upgrade should avoid use of or turn off the trust option, or set it to forbid \htmlData commands, forbid inputs containing the substring "\\htmlData" and sanitize HTML output from KaTeX.

Impact KaTeX users who render untrusted mathematical expressions with renderToString could encounter malicious input using \htmlData that runs arbitrary JavaScript, or generate invalid HTML.

Patches Upgrade to KaTeX v0.16.21 to remove this vulnerability.

Workarounds - Avoid use of or turn off the trust option, or set it to forbid \htmlData commands. - Forbid inputs containing the substring "\\htmlData". - Sanitize HTML output from KaTeX.

Details \htmlData did not validate its attribute name argument, allowing it to generate invalid or malicious HTML that runs scripts.

For more information If you have any questions or comments about this advisory:

- Open an issue or security advisory in the KaTeX repository - Email us at katex-security@mit.edu

Impact

Code that uses KaTeX's trust option, specifically that provides a function to block-list certain URL protocols, can be fooled by URLs in malicious inputs that use uppercase characters in the protocol. In particular, this can allow for malicious input to generate javascript: links in the output, even if the trust function tries to forbid this protocol via trust: (context) => context.protocol !== 'javascript'.

Patches Upgrade to KaTeX v0.16.10 to remove this vulnerability.

Workarounds Allow-list instead of block protocols in your trust function. Manually lowercase context.protocol via context.protocol.toLowerCase() before attempting to check for certain protocols. Avoid use of or turn off the trust option.

Details KaTeX did not normalize the protocol entry of the context object provided to a user-specified trust-function, so it could be a mix of lowercase and/or uppercase letters.

It is generally better to allow-list by protocol, in which case this would normally not be an issue. But in some cases, you might want to block-list, and the KaTeX documentation even provides such an example:

Allow all commands but forbid specific protocol: trust: (context) => context.protocol !== 'file'

Currently KaTeX internally sees file: and File: URLs as different protocols, so context.protocol can be file or File, so the above check does not suffice. A simple workaround would be:

trust: (context) => context.protocol.toLowerCase() !== 'file'

Most URL parsers normalize the scheme to lowercase. For example, RFC3986 says:

Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. An implementation should accept uppercase letters as equivalent to lowercase in scheme names (e.g., allow "HTTP" as well as "http") for the sake of robustness but should only produce lowercase scheme names for consistency.

Impact KaTeX users who render untrusted mathematical expressions could encounter malicious input using \includegraphics that runs arbitrary JavaScript, or generate invalid HTML.

Patches Upgrade to KaTeX v0.16.10 to remove this vulnerability.

Workarounds Avoid use of or turn off the trust option, or set it to forbid \includegraphics commands. Forbid inputs containing the substring "\\includegraphics". Sanitize HTML output from KaTeX.

Details \includegraphics did not properly quote its filename argument, allowing it to generate invalid or malicious HTML that runs scripts.

For more information If you have any questions or comments about this advisory:

Open an issue or security advisory in the KaTeX repository Email us at katex-security@mit.edu

Impact KaTeX users who render untrusted mathematical expressions could encounter malicious input using \def or \newcommand that causes a near-infinite loop, despite setting maxExpand to avoid such loops. This can be used as an availability attack, where e.g. a client rendering another user's KaTeX input will be unable to use the site due to memory overflow, tying up the main thread, or stack overflow.

Patches Upgrade to KaTeX v0.16.10 to remove this vulnerability.

Workarounds Forbid inputs containing any of the characters ₊₋₌₍₎₀₁₂₃₄₅₆₇₈₉ₐₑₕᵢⱼₖₗₘₙₒₚᵣₛₜᵤᵥₓᵦᵧᵨᵩᵪ⁺⁻⁼⁽⁾⁰¹²³⁴⁵⁶⁷⁸⁹ᵃᵇᶜᵈᵉᵍʰⁱʲᵏˡᵐⁿᵒᵖʳˢᵗᵘʷˣʸᶻᵛᵝᵞᵟᵠᵡ before passing them to KaTeX. (There is no easy workaround for the auto-render extension.)

Details KaTeX supports an option named maxExpand which aims to prevent infinitely recursive macros from consuming all available memory and/or triggering a stack overflow error. Unfortunately, support for "Unicode (sub|super)script characters" allows an attacker to bypass this limit. Each sub/superscript group instantiated a separate Parser with its own limit on macro executions, without inheriting the current count of macro executions from its parent. This has been corrected in KaTeX v0.16.10.

For more information If you have any questions or comments about this advisory: Open an issue or security advisory in the KaTeX repository Email us at katex-security@mit.edu

Impact KaTeX users who render untrusted mathematical expressions could encounter malicious input using \edef that causes a near-infinite loop, despite setting maxExpand to avoid such loops. This can be used as an availability attack, where e.g. a client rendering another user's KaTeX input will be unable to use the site due to memory overflow, tying up the main thread, or stack overflow.

Patches Upgrade to KaTeX v0.16.10 to remove this vulnerability.

Workarounds Forbid inputs containing the substring "\\edef" before passing them to KaTeX. (There is no easy workaround for the auto-render extension.)

Details KaTeX supports an option named maxExpand which prevents infinitely recursive macros from consuming all available memory and/or triggering a stack overflow error. However, what counted as an "expansion" is a single macro expanding to any number of tokens. The expand-and-define TeX command \edef can be used to build up an exponential number of tokens using only a linear number of expansions according to this definition, e.g. by repeatedly doubling the previous definition. This has been corrected in KaTeX v0.16.10, where every expanded token in an \edef counts as an expansion.

For more information If you have any questions or comments about this advisory: Open an issue or security advisory in the KaTeX repository Email us at katex-security@mit.edu